1. Field of the Invention
The present invention relates to waterproofing membrane systems. In particular, the invention is directed to an improved method for applying a membrane layer to surfaces such as walls and flat roofs.
2. Discussion of the Background
There are known membranes consisting of multiple layers of bituminous felt (reinforced either with asbestos, fibreglass or polyester), the layers being sealed together with flood pourings of hot roofing asphalt. Membranes traditionally consisted of three, four or five layers of the above materials and were finished with varying surfacing materials including paint, water worn washed river gravel imbedded in a further flood application of roofing asphalt, lightweight asbestos tiles or granolithic concrete topping.
Very early in the development of multilayer membrane systems it was observed that entrapped moisture in the supporting deck material (which typically consisted of structural concrete, lightweight foam concrete, ash concrete or lightweight aggregate concrete) would result in "blisters", i.e. vapour pockets, occurring in the membrane covering if the membrane was bonded firmly to the substrate material. In practical terms it is very difficult to ensure that the substrate is totally dry. With high summer temperatures, moisture (or moisture vapour) in the substrate is drawn to the underside of the membrane material, expands due to the environmental temperature conditions and lifts the membrane from the substrate (referred to as "blistering").
Approximately 25 years ago certain bituminous ventilating underlays were developed (and sold under the trade mark RUBBERVENT by Rubberoid Roofing Limited, U.K.). These underlays allowed for the lateral dispersion of this vapour pressure. Special constructions were used to allow this pressure to escape at parapets, or specially designed vents were incorporated in the roofing system.
The underlay or base layer commonly used in multilayer membrane systems consisted of a standard bituminous felt with a fine granular gravel undersurface to provide escapement channels, with approximately 10 mm diameter perforations occurring at a 75 mm to 100 mm grid frequency.
Small bituminous "rivets" were formed through the above described perforations when bitumen was applied to the upper surface of the base layer for the bonding of the subsequent layers. These rivets provided a small degree of attachment of the membrane to the substrate, but because of the normal dead load of the membrane construction (generally including the weight of the gravel covering or alternatively granolithic paving) this attachment was regarded as a "bonus" rather than the prime reason for the use of this material.
The above described form of ventilation to bituminous membranes has been in common use in Australia for approximately 25 years.
During the past five years, synthetic single layer membranes have become very popular. (Throughout the specification, the term "single layer membrane" has its trade meaning, i.e. a membrane layer suitable for use on its own, with or without an underlay, although further layers can be added).
Single layer membranes fall into many categories, however the most popular are:
1. Butyl PA1 2. Butyl/E.P.D.M. blend PA1 3. E.P.D.M. sheeting PA1 4. P.V.C. sheeting PA1 a. flexibility (stretchability) PA1 b. "cold" application (as opposed to the requirement for hot bitumen for traditional bituminous membranes) PA1 c. speed of installation (a single layer is a lot easier to apply than multi layer traditional construction) PA1 d. approved aesthetic appearance (pre-coloured synthetic sheets are available) PA1 e. ease of inspection for mechanical failure PA1 f. excellent joining properties between sheets which incorporated "welding" procedures or the introduction of uncured base materials. PA1 g. economy (Petroleum based bituminous products have rapidly increased in price because of increases in base petroleum prices). PA1 (a) Application of ballast PA1 (b) Ventilating Strips PA1 (c) Mechanical Anchors
The average thickness of a single layer membrane is between 1 mm to 2 mm as comPared to traditional multilayer membranes which range from 10 mm to 15 mm in thickness (excluding the gravel, tile or granolithic paving surfacing).
The single layer systems exhibit desirable characteristics that were unavailable in relation to multilayer traditional bituminous systems, including:
Because of the lightweight nature of the material of single layer membranes, secure adhesion to the substrate material is required to ensure that negative pressures created by wind, etc. do not induce "lift" in the membrane system.
As with problems experienced in the early development of multilayer bituminous roofing, solid complete adhesion of a single layer membrane to substrates previously mentioned re-introduces the problem of "blistering" to this form of roof membrane.
The glue line (normally a form of contact adhesive) prohibits the lateral movement of vapour occurring between the underside of the membrane and the substrate material and blistering inevitably occurs where the materials are applied over a "wet" form of construction.
Several systems have been developed in an endeavour to eliminate blistering occurring in single layer membrane systems, some of these being:
In this instance the membrane is laid in the form of a pre-formed blanket without any form of adhesion occurring between the membrane material and the substrate, the membrane being held in position by the application of a thickness (normally 75 mm minimum) of washed water worn river gravel.
Lack of adhesion allows for the horizontal dispersement of vapour pressure which can be vented in a traditional manner. However, the membrane is loose and the weight and cost of the river gravel are major disadvantages.
Some manufacturers have developed a system of laying polythene pressure sensitive strips in a grid formation (normally a square pattern of 600 mm to 1000 mm grid dimensions) the intention being that the vapour pressure transmits laterally to these strips and is subsequently ventilated through a parapet or alternate venting system. This system is not regarded as a total answer to the problem as blisters can still occur within the square created by the ventilating strips. Also, considerable costs are involved in the application of these tapes.
Some manufacturers in the U.S.A. (e.g. Carlisle Rubber) have developed a system of mechanical anchors which incorporate plastic clips on a predetermined grid frequency (dependent on the anticipated uplift) with snap on covers which provide mechanical attachment. The major draw back with this system is the stretching/tensioning of the membrane material which can introduce rapid deterioration from normal environmental conditions.